Atmospheric gases, such as oxygen, nitrogen and argon, are generally produced by the separation of air into its constituents. The energy to carry out this separation is generally provided in the form of elevated pressure by the compression of the feed air. One method of compressing the feed air is to pass it through a compressor driven by a gas turbine powered by expanding gas from the air separation. For example, U.S. Pat. No. 4,224,045 Olszewski, et al. discloses a system for reducing the compression energy required by integrating the air separation system with a gas turbine. A portion of the compressed air from the gas turbine air compressor is mixed with fuel and combusted. At some point prior to expansion, compressed nitrogen from the lower pressure column of a double column cryogenic air separation plant is added to the combustion mixture, and the resulting gaseous mixture is expanded in a power turbine. The expansion provides energy to compress the feed air to the double column air distillation process.
A method generally employed to purify feed air of high boiling impurities, such as water, carbon dioxide, and hydrocarbons, prior to separation in the air separation facility, employs the use of reversing heat exchangers wherein these impurities are frozen out of the feed air stream. However, the high operating pressures of integrated gas turbine air separation systems generally exceed the practical pressure limits of commercially available reversing heat exchangers. It is therefore desirable to use adsorbent bed prepurifiers for feed stream purification. U.S. Pat. No. 4,557,735-Pike teaches a method of employing such prepurifiers with integrated gas turbine air separation. This patent teaches cleaning the feed air in prepurifiers containing heat regenerable adsorbent, and regenerating the adsorbent with a portion of the waste nitrogen which has been preheated against hot compressed air from the gas turbine air compressor. However, the hot regeneration gas is required only on an intermittent basis. This leads to fluctuations within the process. When hot air is not required for heating the regeneration gas, the extra air flow must either be added to the main feed air waste nitrogen heat exchanger thus causing fluctuations in outlet temperature for both air and nitrogen, or it must be cooled in a separate heat exchanger against some medium such as cooling water thus adding to the capital requirements for the system. Furthermore, because regeneration gas is added to the main waste nitrogen stream prior to compression, temperature variations in the nitrogen compressor feed due to variations in regeneration gas temperature may cause operational problems with the nitrogen compressor.
It is therefore an object of this invention to provide an air separation method employing an integrated gas turbine and heat regenerable adsorbent purifiers wherein temperature and flow variations for feed air and return nitrogen streams are substantially reduced.